Present day automatic, and semi-automatic operation of railroad vehicles divides the railroad track into units called blocks. Wayside circuitry is capable of resolving vehicle location to within a block. To provide vehicle carried apparatus (or an operator, if one is present) with traffic information, other wayside circuitry transmits an indication of the distance between a vehicle and the immediately preceding vehicle. This information is coded so as to represent, at least, a speed limit. The speed limit is computed such that it is always possible for the following vehicle to stop within the unoccupied distance between vehicles. Actually typical operation has a substantial safety factor since the speed limit is calculated on the assumption that the following vehicle is about to exit from the block, and thus, the assumed clear space between vehicles is a limiting case which only approaches the reality as the following vehicle approaches a block boundary.
The foregoing analysis is particularly significant in examining typical vehicle operation as it transits or crosses a block boundary. As the vehicle crosses the block boundary the wayside circuitry subtracts an entire block length from the clear distance between vehicles, although at that instant of time there actually exists the greatest safety factor, since the assumed clear distance between vehicles is less than the actual clear distance between vehicles by the length of the block. This typical operation has, for many years, been beset by a problem which manifests itself at the block boundary; typically, the allowable speed limit in the block being entered may be lower (a lower speed limit) than the speed limit (the higher speed limit) in the block being exited. Thus, a train which was quite safely being operated near the higher speed limit can be abruptly placed into an overspeed condition as it crosses the block boundary as the speed limit drops to the lower speed limit.
Typical vehicle carried apparatus includes a governor; the governor has at least two input signals, one representing actual vehicle speed, and the other representing the wayside speed limit; and the governor continually compares these. If actual speed exceeds the speed limit an overspeed condition is detected. So long as an overspeed conditions is not detected, the governor does not (typically) interfere with operation of the vehicle. However, when an overspeed condition is detected the governor may automatically impose a brake application; or signals the vehicle operator that he must impose a brake application, and if he fails to so impose a brake application, the governor may then thereafter automatically impose a brake application. Regardless of the specific operating procedure, because of the ensuing braking operation the governor loses control of the rate of which the vehicle is brought under the new speed limit. Either the brake application brings the vehicle under speed sooner than is actually required by the actual clear distance between vehicles, or the vehicle does not decelerate quickly enough. Because block length and associated speed limits are calculated based on worst case analyses it is apparent that a higher speed limit could be tolerated at the entering end of a block than is now possible under present day operating procedures and with present day operating apparatus.
Another application which generates a similar problem is that of station stopping. Currently, as a vehicle is brought into a station under manual control the only apparatus enforced speed limit is a low speed limit of say 15 to 35 miles per hour. Even automatic station stopping apparatus employs a non-zero speed limit. Since the speed limit is non-zero, vehicle headway must allow for the possibility of the vehicle overrunning the station in the worst case.
Both of these (and other) situations could be improved by the generation on board the vehicle of a speed limit which transited smoothly from an upper to a lower speed limit. To alleviate these problems the speed profile must have two characteristics; firstly, it gradually decreases, say from a first or higher speed limit when the profile generation apparatus is initiated, to a lower or zero speed limit at the termination of the speed profile generation. The second characteristic is that of vitalness; that is, the speed profile can be depended upon to ensure vehicle safety.
The prior art does evidence a vehicle carried apparatus to generate a profile speed limit, one that decreases with time and/or distance. However, typically as is illustrated in Macano, U.S. Pat. No. 3,934,125, the profile speed limit is not vital in that it is associated with a higher speed limit on which safety is predicated. Such an arrangement would, of course, not at all meet the needs of the situation referred to above. What is required, is a vital speed profile which can be depended upon for safety. For example, one that could be fed to the governor, along with the wayside generated speed limit, and safely allow the governor to control the vehicle to be at a speed below the higher of the two (speed profile or wayside generated) limits.
Reference to FIGS. 1-3 will help explain the deficiencies is presently used equipment and the result of using either of the two different embodiments of the invention.
FIG. 1 illustrates a profile of vehicle speed versus some monotonically increasing parameter such as time and/or distance. The horizontal lines in FIG. 1 represent both an "old" speed limit and a "new" speed limit. Since the problems sought to be overcome arise from a decrease in speed limit, the relationship shown in FIG. 1 between the "old" and "new" speed limits is one wherein the "old" speed limit is the higher of the two. Besides these speed limits FIG. 1 also represents actual vehicle velocity. In FIG. 1 the "old" speed limit is effective for values of the abscissa parameter less than A.sub.1 and the "new" speed limit is effective for values of the abscissa greater than A.sub.1. This is an accurate portrayal of typical operating conditions inasmuch as the vehicle carried apparatus in many cases has no warning of an impending reduction in speed limit as occurs at the value of the abscissa corresponding to A.sub.1. Conventional vehicle carried apparatus would (either manually, semi-automatically or automatically) control the actual vehicle speed to lie near but below the "old" speed limit in the region governed by that limit. However at the transition (that is the values of the abscissa near to but larger than A.sub.1) the vehicle is overspeed. Under those circumstances the equipment imposes a braking force or requires the operator to apply the vehicle breaks to bring the actual vehicle speed down, as is shown in FIG. 1. At the time when the actual vehicle speed drops below the "new" speed limit (at an abscissa value corresponding to A.sub.2) the brake application may be removed (either manually or automatically) and the same vehicle carried apparatus will control the speed of the vehicle at but slightly below the "new" speed limit. It is a goal of the invention to smooth the transition from old to new speed limits so that the new speed limit is enforced only at the exit end of the block whose entrance is at A.sub.1. In many cases the brake application can be avoided altogether. In other situations the deceleration can be reduced.
FIGS. 2 and 3, on the other hand show operation in accordance with the present invention. More particularly, FIG. 2 shows speed limit on a vertical axis, and particularly noted on that axis are a "old" speed limit and a "new" speed limit, wherein the transition between the old and the new limits occurs at a value of the abscissa A.sub.1. In the case of FIG. 2 the abscissa represents time, the solid line represents a speed limit profile generated by vehicle carried apparatus at a transition from a higher to a lower speed limit. In the case of the operation shown in FIG. 2, time is broken up into a number of segments. R.sub.1 -R.sub.4. Associated with each segment is a speed reduction rate (for the case of FIG. 2 it is r.sub.n in miles per hour/second). For each segment the speed profile provides a speed limit corresponding to the speed limit at the beginning of the segment less the product of the associated rate (r.sub.n where n identifies the segment) multiplied by the current extent of travel through the segment. Thus as is shown in FIG. 2 velocity reduction is minimal in the first segment, and it increases sequentially through the second, third and fourth segments. This increase in the velocity reduction rate is indicated by the increase in slope of the line representing the speed profile. Vehicle carried apparatus is provided with the speed profile so that the vehicle speed is (manually, semi-automatically or automatically) controlled to lie near but below the effective speed profile.
FIG. 3 is a similar representation except that now the abscissa represents distance rather than time and thus the four different segments represent vehicle travel rather than time periods. Similarly, the reduction rate (r) is measured in miles per hour/foot.
It is significant to note that both in FIGS. 2 and 3, notwithstanding that the wayside imposed speed limit (the new speed limit), is below the actual train speed for most of the durations shown in FIGS. 2 and 3, the vehicle is not necessarily subjected to braking (either equipment applied or an operator required application) if the vehicle speed is below the speed profile. This operation is a significant feature of the invention in allowing the vehicle operation to proceed even though the vehicle speed exceeds a wayside imposed speed limit, so long as vehicle speed is less than a vehicle generated speed profile.
In accordance with the invention the vehicle carries a transitional speed limit (profile) generator which is initiated into operation when vehicle carried apparatus detects a wayside imposed speed limit transition from a first or higher speed limit to a second or lower speed limit. In general, the transitional speed limit or speed limit profile comprises a series of monotonically decreasing speed limits which is iteratively generated in the following fashion:
Beginning at an old or higher speed limit and depending on the independent parameter (either distance or time, for example) obtain a product between an effective velocity reduction rate with the change in the independent parameter;
Reduce the initial speed limit by the product obtained above;
Repeat the foregoing steps until either a segment (of time or distance) over which the rate is effective expires or the resulting speed limit is found to be below the newly imposed or lower speed limit.
It is a significant feature of the invention that the foregoing is achieved without compromising vehicle safety.